In recent years, there has been required a heat-resistant alloy excellent in strength and ductility which is suitable for prolonging the life of a plastic working tool for use in a high-temperature environment, such as a hot extrusion die, a seamless tube manufacturing piercer plug, or an injection molding hot runner nozzle.
For this requirement, conventionally, molybdenum (Mo) which is relatively easy to obtain and is excellent in plastic workability and heat resistance has been cited as a candidate. However, in the case of a pure molybdenum material to which no specific element is intentionally added, it cannot be said to be a material suitable for the above-mentioned use because its strength is low.
Accordingly, the strength of a molybdenum material is required to be improved.
As a method of improving the strength of the molybdenum material, there is known a method of adding a different kind of material to molybdenum.
As the method of adding the different kind of material, there is well known a method of adding carbide particles such as TiC particles (Patent Document 1).
On the other hand, in this Mo-carbide two-phase alloy, because of its activity, giant columnar crystals are often formed by abnormal grain growth of the added carbide. For example, in the case of the Ti carbide, the Ti carbide added to Mo forms a solid solution with Mo, wherein the Ti carbide has a TiC particle inside, forms a thin (Mo, Ti) C solid solution phase around the particle, and further forms strong bonding to a Mo phase, which is known as a so-called cored structure (Non-Patent Document 1). However, TiC has a wide nonstoichiometric composition range of C/Ti=0.5 to 0.98. Therefore, the compositions and thicknesses of (Mo, Ti) C intermediate phases differ from each other so that when the (Mo, Ti) C intermediate phases are brought into contact with each other, the grain growth may occur due to stabilization by rediffusion of the respective elements.
The presence of such giant columnar crystals may be a major cause for reduction in strength. It is difficult to control the presence, size, and so on of such giant columnar crystals, thus leading to variation in the strength of the entire material. Also in the case of Zr or Hf which is an element in the same group as Ti, its carbide has crystal structure and nonstoichiometric composition ranges similar to those of TiC and thus forms giant columnar crystals like TiC as described above.
On the other hand, there is also known a method of adding an intermetallic compound of molybdenum as an additive.
As such an intermetallic compound, there is known a Mo—Si—B-based intermetallic compound (e.g. Mo5SiB2) which is an intermetallic compound of molybdenum, silicon, and boron. There is known a method of adding this intermetallic compound to molybdenum, thereby significantly improving the strength in high temperatures (Patent Document 2, Patent Document 3).
This is caused by the fact that Mo5SiB2 has a high hardness. If only the strengths are compared, the material added with Mo5SiB2 is a material much superior to that of Patent Document 1.
However, if high-hardness Mo5SiB2 is added to Mo, the ductility becomes extremely low particularly at 1000° C. or less and becomes approximately zero at room temperature.
Therefore, there has been a problem that the material added with Mo5SiB2 cannot be said to be a material which is also excellent in ductility over a wide temperature range so that its use is limited.